An in-situ uranium assay logging system has been developed that measures directly and quantitatively the uranium concentration in the formation surrounding a drill hole. System operation is based on the DFN (delayed fission neutron) method which involves (1) bombarding a formation with short duration bursts of neutrons from a pulsed-neutron generator to induce fission in any uranium present; (2) separating delayed fission neutrons, from source and prompt fission neutrons, by waiting a few milliseconds after each neutron burst before activating the neutron counter system; (3) deactivating the neutron counter system before the beginning of the next neutron burst; and (4) repeating this bombard-wait-count cycle a sufficient number of times to accumulate a statistically acceptable number of delayed neutron counts. The DFN logging method has been used routinely in our field operations for several years, and it has successfully overcome one of the most perplexing problems encountered in uranium exploration and production--that of radioactive disequilibrium. The need for coring and chemical assaying is virtually eliminated. The information provided by chemical assay of cores is made available by DFN assays on site and in minutes after the data are accumulated. The disequilibrium ratio is available to the field geologist immediately by comparing DFN assay ore grade to ore grade from a calibrated natural gamma-ray log. A DFN uranium assay logging system is capable of 8 ft/minute continuous semiquantitative logs and stationary quantitative assays. Calibration procedures and a method for correcting DFN assays for variable formation parameters, such as porosity and macroscopic absorption cross-section, have been developed. DFN logs, assays, and chemical assays of cores are in excellent agreement.